Modern astronomy and high altitude imaging in general make use of telescopes with large or very large mirrors to enhance optical resolution and light collection efficiency. The high resolution provided by these telescopes is limited by the distortion in the wavefront of the light arriving from celestial objects caused by atmospheric turbulence.
In order to compensate for this distortion, adaptive optics can be employed to correct the wavefront. Such adaptive optics require that a “reading” of the atmosphere be performed in order to characterize the distortion and adjust the optics of the telescope accordingly. One approach has been to use a laser (so called guide star laser) at a wavelength of 589 nm to excite sodium atoms in the atmosphere which induces fluorescence emission. The fluorescence emission from this artificial “star” is then captured and imaged by the telescope. A portion of the imaged light is directed to a wavefront analyzer which analyzes the wavefront and provides the necessary information to update the deformation of the telescope's adaptive optics mirror to eliminate any developing wavefront distortion.
This approach requires high power lasers to enable the light beam to reach altitudes in the 100-km range and to excite a large enough number of the sodium atoms present in an atmospheric layer with a thickness of several km to produce sufficient fluorescence emission. Efficient excitation of the sodium atoms to produce a useful fluorescence emission depends critically on a narrow excitation linewidth and the polarization state of the exciting light. Until very recently, high-power 589-nm guide star laser designs were expensive, bulky, difficult to use and maintain and so delicate and sensitive that they needed to be placed in a clean-room environment far from the launch telescope, necessitating a long and lossy beam relay optics path from the laser to the launch telescope. The breakthrough demonstration of high-power narrow-band Raman amplifiers by researchers at the European Southern Observatory [see for instance, L. Taylor et al., Optics Express 17(17), 14687-14693 (2009) and Y. Feng et al., Optics Express 17(21), 19021-19026 (2009)] proved the feasibility of guide star lasers based on frequency doubling the 1178-nm output of a narrow-band Raman fiber amplifier (RFA). The compact and robust all-fiber nature of such a system means that the laserhead, which includes the RFA and frequency-doubling optics, can be mounted right on the telescope centerpiece structure [see for instance, W. G. Kaenders et al., Proc. SPIE 7736, 232 (2010)] with the remainder of the guide star system (the fiber laser to pump the RFA, the 1178-nm seed laser, wavelength meter, control electronics and the power supplies) housed in an electronics cabinet.
While there are advantages to this frequency-doubled narrow-band 1178-nm RFA system concept, there are challenges to be overcome with regard to the requirements of many telescopes relating to the physical locations of the laserhead and the electronics cabinet.